It is common today to employ tubular filters in a variety of industrial filtering applications. For example, such tubular filters may be employed in mine water draw down procedures during which arsenic and other hazardous materials are filtered out of waste water originating from gold mining operations. Tubular filters also may be employed in waste water pretreatment facilities to filter hazardous materials, such as chromium for example, from industrial waste water.
Tubular filters generally comprise a cylindrical sock of filtration media which is slid over an inner perforated support tube. When a pressure differential is established across the filter, fluid flows through the filtration media and filtrate is removed. In order to prevent by-passing of unfiltered fluid around the sock, it is important that the ends of the sock are tightly sealed to the ends of the tube to avoid leakage of solids through the ends of the sock.
A major improvement in the field of filter media has been the development of expanded polytetrafluoroethylene (PTFE) membrane products. Such membrane products can be produced in a known manner, such as in accordance with the teachings of U.S. Pat. No. 3,953,566, which is incorporated herein by reference. This expanded PTFE comprises a microporous structure, defined by a matrix of nodules interconnected by fibrils, which can be adapted to provide specific filtration properties in many different forms. Socks for tubular filters are commercially available from W. L. Gore & Associates, Inc., Elkton, Md., under the trademark GORE-TEX.RTM. Membrane Tubular Filter Socks. These products comprise an expanded PTFE membrane attached to a felt-like material ranging in many thicknesses, such as but not limited to, thicknesses ranging from about 0.75 to about 2.5 mm. For certain applications, this material may comprise a composite, with additional layers applied to the filtration media for improved durability or better filtration.
Because of the relative inelasticity of most filter media, and in particular filter media comprised at least in part of expanded PTFE, in order for a sock to be positioned over a sock support element, socks for tubular support elements must be dimensioned slightly larger than the outer diametral dimension of a sock support element. However, during use, pressure is applied on the exterior surface of the filter sock, or a vacuum pulls an interior surface of the filter sock, which causes the excess material of the oversized filter sock to detrimentally wrinkle. Such wrinkling compacts the pores of the membrane, which reduces the filtering effectiveness of the filter sock. Additionally, this wrinkling causes premature failure of the filter media.
The foregoing illustrates limitations known to exist in present tubular filter devices. Thus, it is apparent that it would be advantageous to provide an improved tubular support element directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.